This invention relates to a semiconductor laser device, and more particularly to improvements in a surface-emitting injection type laser device.
Semiconductor laser devices include current injection type laser diodes, utilizing a pn junction, which utilizes pumping due to the irradiation with electron rays or light and so on. Among them the laser diodes have been alreadily utilized in many systems as practical devices because the laser diodes have a high power efficiency and can be easily used. These laser diodes are required to effect the laser oscillation in the transverse dominant mode for almost all services. The laser oscillation in the transverse dominant mode has a distribution of light intensities in a plane perpendicular to the direction of propagation of light including a single lumped peak of light intensity on an associated optical axis and a light intensity on each side of the optical axis steadily decreased with an increase in distance from the optical axis in that plane. Accordingly it is extremely important to materialize the transverse dominant mode in practical laser devices.
In laser devices of the type emitting light through the plane of cleavage thereof there have been heretofore established techniques of effecting the laser oscillation in the transverse dominant mode by providing a distribution of refractive indices or a distribution of gains on laser chips involved. In surface-emitting injection type laser devices, however, there has not been discovered a measure to produce the transverse dominant mode in practical devices. This is because the semiconductor wafer has the main face orthogonal to the associated optical axis so that, from the standpoint of the manufacturing process it is technically difficult to provide a distribution of refractive indices or a distribution of gains on the particular laser chip.
A conventional surface-emitting injection type semiconductor laser device has comprised an n type indium phosphide (InP) substrate, an undoped active layer formed of a combination of indium, gallium, arsenic and phosphor (InGaAsP) whose composition is selected to be smaller in forbidden band width than indium phosphide (InP) and disposed on one of the main opposite faces of the substrate, a p type indium phosphide (InP) layer disposed on the active layer, an electrically insulating film with a central window disposed on the p type indium phosphide layer, and an anode electrode disposed on the electrically insulating film and within the window. Then a cathode electrode with a central window has been disposed on the other main face of the substrate with the central window opposed to that on the electrically insulating film and a dielectric film has been disposed within the window on the cathode electrode and on that portion of the electrically insulating film adjacent to the window to increase the refractive index of the other main face.
When a forward voltage is applied across the anode and cathode electrodes, an injection current flows to that portion of the cathode electrode contacted by the other main face of the substrate from that portion of the anode electrode contacted with the p type indium phosphide layer to make the carrier density on the central portion of the active layer high until the so-called population inversion is reached resulting in the simulated emission of photons. Then light is amplified by means of the simulated emission while receprocating between the surface of the p type indium phosphide layer and the other main face of the substrate forming a resonator therebetween. This has resulted in a laser oscillation. Light originating from the laser oscillation has partly passed through the dielectric film to be emitted externally.
In the conventional laser device as described above, a semiconductor wafer or a laser chip formed of the n type substrate, the active layer and the p type layer has a refractive index uniform in a direction perpendicular to the optical axis along which the light reciprocates and the central portion of the active layer has produced thereon a distribution of carrier densities enhanced due to a form factor but the enhanced distribution of carrier densities is not effective for controlling the transverse dominant mode. Although a weak mode selectivity is developed resulting from the form factor that the dominant mode is apt to be excited because of the fact that the surface of the p type layer and the other main face of the substrate form a pair of parallel plane mirrors and only the central portion of the active layer forms a simulated emission region, the thickness of each of the substrate and respective layers is not actually perfectly uniform, and the distribution of carrier densities varies dependent upon the light intensity which is inevitably attended with a change in distribution of refractive indices. Conventional laser devices do not have the mode selectivity capable of withstanding the imperfectness of the thickness and changes in distributions of carrier densities and refractive indices. Accordingly there has been the disadvantage that the controlled transverse dominant mode is not actually obtained.
Accordingly it is an object of the present invention to provide a new and improved surface-emitting injection type semiconductor laser device which can be easily manufactured by existing manufacturing processes and which is operative in the transverse dominant mode.